I'm confused.
Tiber Septim was a mortal, and had nothing to do with the eight divines until he died, thus becoming one.
Lorkhan was a different Aedra from Akatosh entirely, Akatosh being the first Aedra and Lorkhan being the creator of the mortal plane.
Alduin was also the first son of Akatosh, not his incarnation or anything like that.
The current Dragonborn is also not an incarnation, and may not even be related to, Tiber Septim.

Both Tiber Septim and the Dragonborn are Shezzarine, which pretty much means "Avatar or Champion of Shezzar" You may know Shezzar by his other names of Shor or Lorkhan. Also, Alduin and Akatosh being seperate entities wasn't the case until Skyrim, when they needed to keep their pantheon simple while not making Akatosh evil. That's all I can say, but I know TES lore is way more complicated then the games make it out to be. I saw a forum dedicated to Elder Scrolls lore once and it was ******* incomprehensible.